Just about to buy some undercoating for the rear rear wheel wells (mainly) of my latest project, and noticed that some advertise as a sound deadener as one of their effects/benefits. I'm looking at getting a quart of the KBS products (no affiliation), they have a rubberized solvent based product and an asphalt solvent based product - I'd like some opinions on which to go with..
https://www.kbs-coatings.com/undercoat-solv...rubberized.html
https://www.kbs-coatings.com/undercoat-solvent-asphalt.html

So, the other thing I was wondering is, who's used such a product on the rear exterior firewall? These are paintable and I like the idea of using it there, because I personally hate the idea of adding a loose rubber sheet like is original to these cars. I suppose for even greater sound deadening though, you could actually do the exterior and interior of the firewall as the interior gets covered by the pad anyway.. as well as the lower exterior area (where the cable tubes come though).

Think this is a good idea, bad.. Thoughts?


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I’m using 3M Dynatron Dyna-Pro 544 and I will be spraying a smooth coat over primer on the engine side firewall and also using a firewall pad made of modern closed cell foam vs the old “wool” type that is actually original to the cars.

For sound deadening properties, whichever one has the highest cured mass is probably going to perform best. Probably the asphalt.

If you are thinking of applying either of these on the engine side of the firewall, I personally wouldn't. First, you want to make sure whatever you use is rated for the heat in there. You don't want to wind up with an asphalt fire on your hands.

Second, the thick pad on the engine side of the firewall is designed to deaden a different kind of sound than undercoating will deaden. Undercoating and Dynamat type sound deadeners add mass to stop the vibration of panels from transmitting noise. The thick pad deadens the airborne engine noise. I've been extremely happy with this pad made from modern sound deadening material on my firewall. Very lightweight and quiet. I measured 1 dB reduction compared with less expensive foil faced closed cell foam sold for under hood insulation. Wish I had gotten a measurement of no pad at all, but had a brain fart that day and forgot.

Of course, you can always apply deadener to the firewall under the mat like @Van B . I did the same but used Noico constrained layer mat (similar to Dynamat). Even the wife admits the car is very quiet to drive.

Undercoating will essentially provide zero noise reduction as compared to the mat referenced by bbrock in the engine compartment (airborne noise >100 hz) and a constrained layer damping (Dynamat) on the interior bulkhead to address the lower frequency structure borne noise that requires mass damping.